Light guide plate and a method of manufacturing thereof

ABSTRACT

A light guide plate ( 100 ) has an input face ( 11 ) to couple light (B 0 ) emitted by a light source ( 50 ) into said light guide ( 100 ), and an out-coupling grating ( 30 ) to couple light (B 2 ) out of said light guide ( 100 ). Said out-coupling grating ( 30 ) is substantially perpendicular to said input face ( 11 ). A rough cut side of a light guide ( 100 ) is processed by a heated surface processing member ( 701 ) to smooth out irregularities and/or to implement a further diffraction grating ( 12 ) on said input face ( 11 ). The efficiency of coupling light out of the light guide ( 100 ) may be substantially increased and/or stray light effects may be reduced.

FIELD OF THE INVENTION

The present invention relates to light guides, and to methods for makinglight guides.

BACKGROUND OF THE INVENTION

Planar waveguides are cost-effective devices to provide lighting fore.g. liquid crystal displays or key sets. Light initially provided e.g.by an a light emitting diode (LED) may be distributed to a larger areaby means of a planar waveguide. The use of thin planar waveguides mayfacilitate reducing size, weight and manufacturing costs of a portabledevice.

US patent application US2006/0002675 discloses a light guide platecomprising an upper cladding film, core films formed with V-cut grooves,and a lower cladding film. The V-cut grooves may be formed by means of ahot embossing process. Referring to FIG. 10 of US2006/0002675, lightincident on the side surface of the light guide plate propagates in thecore films, and is subsequently vertically reflected from the V-cutgrooves.

SUMMARY OF THE INVENTION

The object of the invention is to provide a light distributing device. Afurther object of the invention is to provide a method of manufacturinga light distributing device.

According to a first aspect of the invention, there is provided amanufacturing method according to claim 1.

According to a second aspect of the invention, there is provided amethod of distributing light according to claim 9.

According to a third aspect of the invention, there is provided a lightdistributing device according to claim 11.

According to a fourth aspect of the invention, there is provided adevice according to claim 14, said device comprising a key set.

According to a fifth aspect of the invention, there is provided a lightdistributing device according to claim 15.

According to a sixth aspect of the invention, there is provided a meansfor distributing light according to claim 16.

The light distributing device comprises a substantially planar waveguidehaving an out-coupling grating and a smoothed or embossed input face,wherein said input face is substantially perpendicular to saidout-coupling grating.

Manufacturing of a planar light guide by die-cutting from a plasticsheet or carrier typically results in an optically diffusing side face.A die-cut side face of a light guide is processed using a hot surfaceprocessing tool. The surface of the side face may be polished by using apolished surface processing member, or the surface of the side face maybe embossed using a member which has a microstructure.

A light beam emitted by a light source is coupled into the light guidethrough the side face to form a second light beam which is waveguided inthe light guide by total internal reflections. The second light beam issubsequently coupled out of the light guide by an out-coupling gratingin order to illuminate e.g. a liquid crystal display or a keypad.

According to the invention, topological errors of the input face may bereduced and/or completely eliminated. By removing topological errors,e.g. defects, the efficiency of coupling light of the light source intothe light guide may be increased. Adverse stray light may be reduced byeliminating light-scattering defects.

Efficiency of coupling light out of a light guide by binary gratingstypically degrades at large angles of incidence. The angle of incidenceat the out-coupling grating may be reduced by implementing refractiveand/or diffractive structures on the input face. The reduction in theangle of incidence may lead to increased efficiency of coupling lightout of the light guide.

The throughput efficiency may be increased by adding diffractive orrefractive structures to the input face of the light guide. The inputgrating or prisms may change the direction of light rays which wouldotherwise propagate substantially straight through the light guidewithout impinging on the out-coupling grating. Thus, the diffractive orrefractive structures may reduce or completely eliminate the portion ofthe light which would propagate substantially straight through the lightguide without impinging on the out-coupling grating. The throughputefficiency means the ratio of optical output power coupled out by theout-coupling grating to the optical power of a light beam impinging onthe input face.

Also the number of interactions between an in-coupled light beam and theout-coupling grating may be increased by diffractive or refractivestructures implemented on the input face. The increased number ofinteractions may also lead to an improved throughput efficiency.

In an embodiment, a substantially collimated output beam may be providedwhen using a substantially collimated light source.

In an embodiment, a thin illuminated keypad and/or a thin illuminateddisplay may be implemented.

The embodiments of the invention and their benefits will become moreapparent to a person skilled in the art through the description andexamples given herein below, and also through the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following examples, the embodiments of the invention will bedescribed in more detail with reference to the appended drawings inwhich

FIG. 1 shows, in a three dimensional view, a light guide and a lightsource adapted to provide back-lighting for a display,

FIG. 2 a shows, in a three dimensional view, a substrate sheetcomprising a plurality of out-coupling gratings,

FIG. 2 b shows, in a side view, cutting of the input face of a lightguide,

FIG. 2 c shows, in a three dimensional view, a rough surface of thedie-cut side face,

FIG. 3 a shows, in a three dimensional view, surface processing of thecut side face,

FIG. 3 b shows, in a side view, surface processing of the cut side face,

FIG. 3 c shows, in a side view, a light guide having a diffractivegrating on its input face, and a corresponding embossing surfaceprocessing member,

FIG. 3 d shows a light guide having refractive prisms on its input face,and a corresponding embossing surface processing member,

FIG. 4 shows, in a side view, the grating period and the grating heightof the in-coupling grating,

FIG. 5 shows, in a side view, light rays emitted from a light source,

FIG. 6 shows propagation of light in the light guide when the input facehas a smooth and flat surface,

FIG. 7 a shows, in a side view, propagation of light in the light guidewhen the input face has an in-coupling grating,

FIG. 7 b shows propagation of light in the light guide when the inputface has an in-coupling grating to diffract light into two differentdirections.

FIG. 7 c shows, in a side view, propagation of light in the light guidewhen the input face comprises prisms,

FIG. 8 a shows angular distribution of the intensity of light emitted bya substantially collimated light source,

FIG. 8 b shows angular distribution of the intensity of light impingingon the out-coupling grating when light is provided by the light sourceof FIG. 8 a and the input face does not comprise a grating structure.

FIG. 8 c shows angular distribution of the intensity of light impingingon the out-coupling grating when light is provided by the light sourceof FIG. 8 a and the input face comprises a grating structure.

FIG. 9 a shows angular distribution of the intensity of light emitted bya slightly collimated light source,

FIG. 9 b shows angular distribution of the intensity of light impingingon the out-coupling grating when light is provided by the light sourceof FIG. 9 a and the input face does not comprise a grating structure.

FIG. 9 c shows angular distribution of the intensity of light impingingon the out-coupling grating when light is provided by the light sourceof FIG. 9 a and the input face comprises a grating structure.

FIG. 10 shows, in a top view, an in-coupling grating adapted tocollimate light in the horizontal direction,

FIG. 11 a shows, in an end view, an input grating 12 which comprisesvertical diffractive ridges to collimate light of the in-coupled beam inthe horizontal direction,

FIG. 11 b shows, in an end view, a crossed grating adapted to collimatelight of the in-coupled beam in the horizontal direction and adapted tochange the direction of the in-coupled beam in the vertical direction,

FIG. 11 c shows, in a three dimensional view, a portion of the surfaceof a crossed grating,

FIG. 12 shows, in a three dimensional view, a surface processing roll,

FIG. 13 shows, in a three dimensional view, a portable device having adisplay and a key set, wherein said display and said key set areilluminated using light guides, and

FIG. 14 shows, in a three dimensional view, a portable device having akey set, wherein the switches of said key set are located under a lightguide.

DETAILED DESCRIPTION

Referring to FIG. 1, a light guide 100 may comprise a substantiallytransparent substrate 10, a substantially planar input face 11 and asubstantially planar surface 18. The light guide 100 may comprise twosubstantially planar and substantially parallel surfaces to implement aplanar waveguide. The light guide 100 may comprise a diffractiveout-coupling grating 30 implemented on the planar surface 18.

An input beam B0 provided by a light source 50 may be coupled into thesubstrate 10 through the input face 11 to form an in-coupled light beamB1 propagating in said substrate 10. The light of said in-coupled beamB1 may be coupled out of the substrate 10 by the diffractiveout-coupling grating 30 to form an output beam B2. The output beam B2may be used e.g. to light a display 220. The output beam B2 may beviewed e.g. by a human viewer (not shown).

The input beam B0 may be provided by a light source 50 which may be e.g.a light emitting diode (LED), a resonant cavity LED, or a laser. Thelight source 50 may be in contact with the input face 11 or at somedistance from it.

The direction SX refers to the initial average direction of the inputbeam B0. If the beam B0 is symmetric, the direction SX is parallel tothe centerline of the beam B0. The out-coupling grating 30 is in ahorizontal plane defined by the directions SX and SY. The horizontaldirection SY is perpendicular to the direction SX. The verticaldirection SZ is perpendicular to the directions SX and SY.

The input face 11 may be substantially perpendicular to saidout-coupling grating 30.

The ratio of the length L1 of the light guide 100 to the thickness t1 ofthe light guide 100 may be greater than 10. The ratio of the width W1 ofthe light guide 100 to the thickness t1 of the light guide 100 may begreater than 10. The thickness t1 of the planar waveguide may be e.g. inthe range of 0.2 to 1 mm. In order to implement light and/or flexiblestructures, the thickness t1 may be in the range of 0.1 to 0.2 mm. Inorder to implement very light and/or flexible structures, the thicknesst1 may be in the range of 0.05 to 0.1 mm.

In order to implement e.g. light distributing device 100 to illuminate akey set and/or display (FIG. 13), the width w1 and/or the length L1 ofthe planar waveguide may be e.g. in the range of 5 to 100 mm. The lightguide 100 may comprise one or more out-coupling gratings 30. The sum ofthe areas of the out-coupling gratings 30 may be e.g. greater than 5% ofthe one-sided area of the planar surface 18.

A waveguiding core of the light guide 100, and in particular one or moreof its planar surfaces may be covered by a cladding layer which haslower refractive index than said core. The cladding may comprise e.g.fluoropolymer, in particular polytetrafluoroethylene.

The dimensions t1, W1, and L1 refer to the dimensions of the waveguidingcore of the planar waveguide 100, i.e. a possible cladding layer is nottaken into consideration.

Referring to FIG. 2 a, the light guide 100 may be manufactured byimplementing a plurality of out-coupling gratings 30 on a substratesheet 900, and cutting the light guide 100 apart. In particular, theinput face 11 may be produced by cutting along a line LL1 shown in FIG.2 a.

The out-coupling gratings 30 may be implemented before said cutting,substantially simultaneously with said cutting, or after said cutting.The out-coupling gratings 30 may be implemented e.g. by embossing.

The material of the sheet 900 may be substantially transparentthermoplastic polymer, e.g. polycarbonate, polymethylmetacrylate (PMMA)or polyvinyl chloride.

Referring to FIG. 2 b, the input face 11 may be cut using a cutting edge901 and a counterpart 902. The cutting edge 901 may be moved in aopposite to the direction SZ with respect to the counterpart 902 inorder to separate a piece 19. The cutting edge 901 may also be arotating cutting disc. The piece 19 may be another light guide 100 or awaste cutting.

Referring to FIG. 2 c, the cutting process may result in surfaces whichare relatively rough when compared to the wavelengths of guided light. Arough input face 11 may refract and scatter the light of the in-coupledlight beam B1 such that harmful stray light effects may arise and/or theefficiency of coupling light from the input face 11 to the out-couplinggrating 30 is decreased.

The rough surfaces cause scattering of light into unwanted directions,i.e. stray light. The input face 11 may comprise defects, i.e.light-scattering protrusions and/or recesses. The criterion for a defectmay be e.g. that a defect causes greater than λ/4 distortion in a planarwavefront of light transmitted through the input face 11. The wavelengthλ may be e.g. 550 nm, which corresponds to the green color.

The wavefront distortion depends on the height of a defect and therefractive index difference over the input face 11. The refractive indexof polycarbonate is approximately 1.6, and the refractive index ofpolymethylmetacrylate is approximately 1.5. The respective refractiveindex difference for an air-substrate interface may be e.g. 0.5 to 0.6.Thus, a protrusion of 0.25 μm may cause a 125 to 150 nm retardation in awavefront transmitted through the input face 11. The retardation of 125nm corresponds approximately to λ/4 for the wavelength 550 nm.

For demanding applications, the criterion for a defect may also bedefined such that it causes less than λ/10 distortion in the wavefront,and/or that the defect protrudes more than λ/10 μm from the averagelevel of the input face 11, wherein the wavelength λ may be e.g. 550 nm.

Referring to FIG. 3 a, a rough input face 11 may be processed bypressing a surface processing member 701 against the face 11. Pressingby the surface processing member 701 may smooth out irregularitiesand/or may implement a diffraction grating on the input face 11.

The member 701 may have e.g. a substantially flat polished surface 703to smooth the surface of the input face 11.

The smoothed input face 11 may polished, i.e. it may be substantiallynon-diffusing. The smoothing may reduce the number and the size of lightscattering defects such that they cover less than 5%, or even less than1% of the area of the input face 11.

Referring to FIG. 3 b, the surface processing member 701 may be pressedagainst the input face 11 by an actuator 720 or spring mechanism in thedirection SX with respect to the substrate 10. The substrate 10 may beclamped between a first clamping block 741 and a second clamping block742 to keep it fixed. The actuator 720 may be e.g. a pneumatic,hydraulic or electromechanic actuator

The surface of the input face 11 may be softened by heating in order tofacilitate smoothing and/or embossing. The surface layer of the inputface 11 may be kept at an elevated temperature, e.g. at a temperaturegreater than 150° C. during the processing in order to facilitate theprocessing. The material of the substrate 10 may have a softeningtemperature, which is also known as the glass transition temperatureT_(G). For example, the glass transition temperature of polycarbonate istypically in the range of 145 to 150° C., and the glass transitiontemperature of polymethylmetacrylate is approximately 105° C. During theembossing or smoothing process, the temperature T_(s) of the surface ofthe input face 11 should reach at least temporarily a maximumtemperature which is greater than T_(G).

A first heater 730 may be adapted to heat the surface processing member701. The heater 730 may be e.g. an electrical heating element or a heatexchanger for transferring heat from hot fluid to the member 701. Asecond heater 740 may be adapted to heat the input face 11 prior to theprocessing and/or during the processing. The heater 740 may be basede.g. on infrared radiation or hot gas flow.

The surface processing member 701 may exert an embossing pressure on theinput face 11. The embossing pressure may be substantially equal to apredetermined value.

The maximum temperature T_(s) of the input face 11 may be kept lowerthan a predetermined upper limit in order to avoid boiling of thesubstrate material, in order to avoid irreversible chemical damage ofthe substrate material, in order to minimize sticking of the substratematerial to the surface processing member 701, and/or in order to avoidexcessive deformation of the input face 11 due to pressure caused by thesurface processing member 701 or deformation due to gravity. Anexcessive deformation may lead e.g. to a local increase in the thicknesst1 and width W1 near the input face 11 during the pressing.

During the embossing or smoothing process, the temperature T_(s) of thesurface of the input face 11 may reach at least temporarily a maximumtemperature, which is e.g. in the range of T_(G) to T_(G)+30° C., in therange of T_(G)+30° C. to T_(G)+70° C., in the range of T_(G)+70° C. toT_(G)+100° C., or even in the range of T_(G)+100° C. to T_(G)+170° C.The use of high temperatures may facilitate implementing of finemicrostructures, but may also require fast heating and cooling of theinput face 11 so that only a thin surface layer is deformed during theprocessing. The maximum temperature T_(s) of the input face 11 may beselected to correspond to a predetermined embossing pressure.

The temperature of the surface processing member 701 may be kept below apredetermined temperature in order to minimize sticking of the substratematerial to the member 701 and/or in order to minimize deformation of anembossed structure when the member 701 is separated from the input face11. However, the temperature of the surface processing member 701 shouldnot be so low as to harden the input face 11 before the embossing iscompleted. During the processing, the temperature of the surfaceprocessing member 701 may be lower than the glass transition temperatureT_(G), and/or the temperature of the surface processing member 701 maybe e.g. 20° C. to 50° C., 50° C. to 100° C., or even 100° C. to 200° C.lower than the maximum temperature of the input face 11.

In particular, the temperature of the surface processing member 701 maybe kept 10 to 30° C. lower than a self-adhesive temperature of thesubstrate material in order to minimize sticking. The self-adhesivetemperature is defined as the minimum temperature at which two layers ofsaid substrate material will mutually adhere when pressed togetherwithout using any release agents. The surface processing member 701 maybe coated with e.g. fluoropolymer-based release agent before theprocessing in order to minimize sticking.

Referring to FIG. 3 c, the surface processing member 701 may alsocomprise a grating structure 702 to emboss an in-coupling gratingstructure 12 to the input face 11. The surface processing member 701 hasa surface relief which corresponds to the surface relief of thein-coupling grating 12.

The grating structure 702 may be implemented e.g. on a nickel shim byoptical methods, electrolytic methods and/or electron beam lithography.

Referring to FIG. 3 d, the surface processing member 701 may have amacroscopic pattern 9 in order to emboss a macroscopic structure 8 tothe surface of the input face 11. The macroscopic structure may comprisee.g. one or more prisms.

Referring to FIG. 4, the in-coupling grating 12 may have a gratingperiod d1. The embossed diffractive features 15 may have a height h1.For visible light, d1 may be e.g. in the range of 0.2 to 2 μm. Theheight h1 may be e.g. 0.25 to 4 times a wavelength of visible light.

A filling factor f is the ratio of the width w2 of the diffractivefeatures 15 compared to the grating period d1. The filling factor of thegratings 12, may be e.g. in the range of 40% to 60%.

A gap between the light source 50 and the in-coupling grating 12 may befilled with a transparent filler, e.g. adhesive. In that case thegrating period d1 and the profile height h1 of the in-coupling grating12 may be selected to be e.g. substantially equal to 2 μm.

The grating period of the surface processing member 701 is selected tobe substantially equal to d1. The height of the embossing microstructureof the member 701 is selected to be equal to or greater than h1.

Referring to FIG. 5, the light source 50 provides an input light beamB0. The light source 50 may comprise e.g. a light emitting diode (LED)and a collimating structure, e.g. a convex lens. The intensity I(θ₁) ofthe input beam B0 may depend on the angle θ₁, where the angle θ₁ is avertical angle between the direction of a light ray LR1 and the surfacenormal N1 of the input face 11. The angular dependency of the intensityI(θ₁) may be expressed e.g. by the equation (1):

I(θ₁)=I ₀ cos^(n)(θ₁)  (1)

Where I₀ is the intensity in the direction SX, and cos_(n) denotescosine to the power of n.

The beam B0 provided by the light source 50 may be substantiallycollimated in the vertical and/or horizontal directions. The verticaldivergence and/or the horizontal divergence may be in the range of 0 to5 degrees, or in the range or 5 to 20 degrees. The beam B0 may beslightly collimated in the vertical and/or horizontal directions, i.e.the vertical divergence and/or the horizontal divergence may be in therange of 20 to 60 degrees. The beam B0 may be highly diverging, and thevertical divergence and/or the horizontal divergence may even be in therange of 60 to 180 degrees.

Referring to FIG. 6, the input beam B0 provided by the light source 50may be substantially collimated and the input face 11 may be flat andoptically smooth. Consequently, the in-coupled beam B1 is alsosubstantially collimated and it impinges, in average, on theout-coupling grating 30 at a large angle θ₂. The angle θ₂ is an anglebetween a light ray and a surface normal N2 of the out-coupling grating30. Consequently, the in-coupled beam B1 interacts only few times withthe out-coupling grating 30, and the efficiency of coupling light out ofthe substrate 10 may be low. It may be that some light rays of thein-coupled beam B1 do not impinge on the out-coupling grating 30 at all,even if they were reflected from the lower planar surface of the lightguide 100.

The embodiment of FIG. 6, i.e. without the in-coupling grating 12, maybe used e.g. when the out-coupling grating 30 is long when compared tothe thickness t1 of the substrate 10.

The angle α between the input face 11 and the out-coupling grating 30may be in the range of 80 to 100 degrees. In particular, the angle α maybe substantially equal to 90 degrees.

Referring to FIG. 7 a, The direction of the beam B1 may be changed bycoupling the input beam B0 into the substrate 10 through an in-couplinggrating 12 implemented on the input face 11. Consequently, thein-coupled beam B0 may impinge, in average, on the out-coupling grating30 at an angle θ₂, which is substantially smaller than in the case ofFIG. 6. The in-coupled beam B1 also interacts more times with theout-coupling grating 30 than in the case of FIG. 6. The number ofinteractions between the light beam B1 and the out-coupling grating 30is inversely proportional to tan(θ₂).

The efficiency of coupling light out of the substrate 10 may besubstantially greater than in case of FIG. 6.

The local coupling efficiency at the left side of the out-couplinggrating 30, i.e. near the input face 11 may be maximized by minimizingthe angle θ₂ of the in-coupled beam B1 but keeping the angle θ₂ greaterthan a predetermined limit in order to fulfill the criterion for totalinternal reflection. The angle θ₂ may be selected e.g. such that no morethan 5% of optical power is coupled out of the lower planar surface ofthe light guide 100. The local coupling efficiency is defined to be theratio of the intensity of the output beam B2 to the intensity of thein-coupled beam B1 at a given point of the out-coupling grating 30.

In order to facilitate coupling of light out of the light guide 100, theangle θ₂ for the average direction of the in-coupled beam B1 may beselected to be greater than three times arctan(t1/L1)

The angle θ₃ between the average direction of the output beam B2 and thesurface normal N2 of the out-coupling grating 30 may be e.g. in therange of 0 to 20 degrees. The output beam B2 may be substantiallyperpendicular to the out-coupling grating 30. The out-coupling grating30 may be substantially planar.

The angle α between the input face 11 and the out-coupling grating 30may be in the range of 80 to 100 degrees. In particular, the angle α maybe substantially equal to 90 degrees.

Referring to FIG. 7 b, the in-coupling grating 12 may be adapted todiffract light into one or more diffraction orders other than zero, e.g.into the diffraction orders minus one and one, in order to increase thenumber of interactions with the out-coupling grating 30 and/or in orderto decrease the angle of incidence θ₂. The in-coupling grating 12 may beadapted to diffract at least 30% of the optical power of the input beamB0 into the diffraction order −1 or 1.

Referring to FIG. 7 c, the input face 11 may comprise one or more prisms8 a, 8 b to direct the in-coupled beam B1 a and/or B1 b away from thedirection of the surface normal N1, in order to increase the number ofinteractions with the out-coupling grating 30 and/or in order todecrease the angle of incidence θ₂. In particular, the input face maycomprise two or more prisms 8 a, 8 b. Increasing the number of theprisms may allow reducing the height of said prisms in the direction SX.Consequently, the light guide 100 may be shorter.

When the input face 11 comprises macroscopic prisms 8 a, 8 b, thesurface normal N1 refers to the surface normal of a tangential plane TP1of the input face 11. The angle α between the normal N1 input face 11and the normal N2 of the out-coupling grating 30 may be in the range of80 to 100 degrees. In particular, the angle α may be substantially equalto 90 degrees.

The angle α between the input face 11 and the out-coupling grating 30may be in the range of 80 to 100 degrees. In particular, the angle α maybe substantially equal to 90 degrees.

The prisms 8 a, 8 b are macroscopic refractive triangular features whichhave at least one face 81 a, 81 b which is inclined with respect to thetangential plane TP1 of the input face 11 in order to re-direct thein-coupled light beams B1 a, B1 b. An angle γ between the faces 81 a, 81b of the prisms 8 a, 8 b and the tangential plane TP1 may be e.g. in therange of 10 to 60 degrees.

Referring back to FIGS. 7 a and 7 c, the fraction of light transmittedsubstantially in the direction of the surface normal N1 may be reduced.Thus, the fraction of light which would otherwise be transmittedsubstantially in the direction of the normal N1 through the substrate 10may also be coupled out of the substrate 10.

The input beam B0 emitted from the light source 50 may have apredetermined vertical divergence in the direction SZ. When the light ofsaid beam B0 is coupled out by the out-coupling grating 30, the outputbeam B2 may have substantially the same divergence in the direction SX.In other words, The output beam B2 may have substantially the samedivergence as the input beam B0.

Embossing of the in-coupling grating 12 may require smaller deformationof the input face 11 than embossing of the prisms 8.

FIG. 8 a shows the angular distribution of intensity I(θ₁) of asubstantially collimated input beam B0 emitted by a light source 50.

Referring to FIG. 8 b, it is assumed that the beam of FIG. 8 a iscoupled into a substrate through a smooth input face 11, as shown e.g.in FIG. 6. The input face 11 represents an air-polycarbonate interface,the angular intensity distribution of the beam B0 is expressed by eq.(1) having n=50, and the wavelength is 630 nm. FIG. 8 b shows theangular intensity distribution of light B1 impinging on the out-couplinggrating 30. It may be noticed that the peak of the angular intensitydistribution is approximately at an angle θ₂ of 85 degrees. The positionof the peak depends slightly on the ratio of the length of theout-coupling grating 30 to the thickness of the substrate 10.

Referring to FIG. 8 c, it is assumed that the beam of FIG. 8 a iscoupled into a substrate through a binary input grating 12 having aperiod d1 of 0.7 μm, a filling factor of 0.5, and a profile height h1 of0.4 μm. The input face 11 represents an air-polycarbonate interface, theangular intensity distribution of the beam B0 is expressed by eq. (1)having n=50, and the wavelength is 630 nm. FIG. 8 c shows the angularintensity distribution of light B1 impinging on the out-coupling grating30. It may be noticed that the peak of the angular intensitydistribution is approximately at an angle θ₂ of 69 degrees.

The diffraction efficiency of a typical binary grating is rather low atlarge angles of incidence. Thus, light may be coupled out of thesubstrate 10 substantially more effectively in case of FIG. 8 c than incase of FIG. 8 b. In case of FIG. 8 c, the efficiency of out-coupling bya binary out-coupling grating 30 is 5.5% for TE-polarization in thediffraction order −1, when the output grating 30 has a binaryrectangular profile, a grating period d1 of 0.43 μm, a filling factor of0.5, and a profile height h1 of 0.25 μm. In case of FIG. 8 b, therespective out-coupling efficiency is only 1% by using the same outputgrating 30.

FIG. 9 a shows the intensity distribution of a slightly collimated inputbeam B0 emitted by a light source 50.

Referring to FIG. 9 b, it is assumed that the beam of FIG. 9 a iscoupled into a substrate through a smooth input face 11, as shown e.g.in FIG. 6. The input face 11 represents an air-polycarbonate interface,the angular intensity distribution of the beam B0 is expressed by eq.(1) having n=10, and the wavelength is 630 nm. FIG. 9 b shows theangular intensity distribution of light B1 impinging on the out-couplinggrating 30. It may be noticed that the peak of the angular intensitydistribution is still approximately at an angle θ₂ of 85 degreesalthough the distribution is broader than in case of FIG. 8 b. Theposition of the peak depends slightly on the ratio of the length of theout-coupling grating 30 to the thickness of the substrate 10.

Referring to FIG. 9 c, it is assumed that the beam of FIG. 9 a iscoupled into a substrate through a binary input grating 12 having aperiod d1 of 1 μm, a filling factor of 0.5, and a profile height h1 of0.5 μm. The input face 11 represents an air-polycarbonate interface, theangular intensity distribution of the beam B0 is expressed by eq. (1)having n=10, and the wavelength is 630 nm. FIG. 9 c shows the angularintensity distribution of light B1 impinging on the out-coupling grating30. It may be noticed that the peak of the angular intensitydistribution is approximately at an angle θ₂ of 73 degrees, and theangular intensity distribution has a considerable value still at anangle 53 degrees.

The input grating 12 or the prisms 8 a, 8 b may be adapted to direct thelight of the in-coupled beam B1, B1 a, B1 b such that an angle betweenthe average direction of light impinging on the out-coupling grating 30and the normal N2 is smaller than 70 degrees, in particular smaller than60 degrees.

Referring to FIG. 10, the input grating 12 may also have diffractivefeatures to diffract the input beam in the horizontal direction. (φ1denotes a horizontal angle between a light ray LR1 and the surfacenormal N1.

Referring to FIG. 11 a, the input grating 12 may comprise a plurality ofsubstantially linear diffractive microscopic ridges 16 or grooves whichare oriented in the vertical direction SZ to diffract light in thehorizontal direction SY. The vertical ridges 16 may have a gratingperiod d2. The vertical ridges 16 may be adapted to collimate thein-coupled beam B1 in the horizontal direction, i.e. the ridges 16 mayact as a diffractive collimator. The ridges 16 may have aposition-dependent grating period d2, i.e. a variable line density as afunction of the horizontal distance y from the light source 50. Thecollimation may require a relatively high accuracy for positioning thelight source 50 with respect to the input face 11.

Alternatively, the input grating 12 may comprise a plurality ofsubstantially linear diffractive ridges 15 (FIG. 4) or grooves which areoriented in the horizontal direction SY to diffract light in thevertical direction SZ. The horizontal ridges 15 or grooves may have agrating period d1 (FIG. 4).

FIG. 11 b shows a crossed grating. The input grating 12 may comprise aplurality of diffractive features 17, which are adapted to diffractlight simultaneously in the vertical direction SZ and the horizontaldirection SY. The diffractive features may be e.g. rectangular or ovalmicroscopic studs (FIG. 11 b). The diffractive features 17 are arrangedalong substantially vertical lines VL in order to diffract light in thehorizontal direction SY and also along substantially horizontal lines HLin order to diffract light in the vertical direction SZ. The verticallines VL are substantially parallel to the vertical direction SZ, andthe horizontal lines HL are substantially parallel to the horizontaldirection SY. The distance between the horizontal lines HL is equal tothe grating period d1, and the distance between the vertical lines VL isequal to a grating period d2. Thus, the horizontal lines HL are arrangedsuch that the input grating 12 has a first grating period d1 fordiffraction in the vertical direction, and the vertical lines VL arearranged such that the input grating 12 has a second grating period d2for diffraction in the horizontal direction.

The position of the vertical lines VL may correspond to aposition-dependent grating period d2, i.e. a variable line density as afunction of the horizontal distance y from the light source 50. Thediffractive features 17 may be adapted to collimate the in-coupled beamB1 in the horizontal direction. In other words, the features 17 may actas a diffractive collimator. The collimation may require a relativelyhigh accuracy for positioning the light source 50 with respect to theinput face 11.

FIG. 11 c shows a portion of a surface relief grating 12 according toFIG. 11 b. The input grating 12 comprises a plurality of protrusions 17which are arranged along a plurality of vertical lines VL and along aplurality of horizontal lines VL. The diffractive features 17 may belocated at the intersections of the vertical lines VL and the horizontallines. The protrusions 17 define a plurality of vertical and horizontalgrooves between said protrusions 17. Alternatively, the input grating 12may comprise a plurality of recesses which define a plurality ofvertical and horizontal ridges between them.

The input grating 12 may be e.g. a slanted grating to diffract amajority of optical power substantially into one direction anddiffraction order, e.g. into the diffraction order one.

The cutting and surface processing operations may be performed as aroll-to-roll process.

Referring to FIG. 12, the surface processing member 701 may also be aroll having a smooth surface or a grating pattern 702 on its surface.

The roll 701 may be pressed against the input face 11, and the inputface 11 may be moved in the direction SY with respect to the roll 701 inorder to emboss a grating pattern to said input face 11.

In an embodiment, the heated cutting member 901 also acts as the surfaceprocessing member 701.

The light guide 100 may be optimized to operate at a predeterminedwavelength λ selected from the range of visible wavelengths 400-760 nm.The light guide 100 may be optimized to operate at the green wavelength550 nm or at the whole range of visible wavelengths 400-760 nm.

The substantially planar surface 18 of the light guide 100 may have oneor more out-coupling gratings 30.

FIG. 13 shows a device 200 comprising a keyset 230 and/or a display 200.One or more light distributing devices 100 may be used to provide frontand/or back lighting to e.g. a liquid crystal (LCD) display 200 or aMEMS display (Micro-Electro-Mechanical System). One or more lightdistributing devices 100 may be used to provide lighting to or for akeyset 230. Partially transparent touch-sensitive elements, switches andor proximity sensors may be positioned on the top of the light guide100, as shown in FIG. 13. The keyset 230 may be a keypad or a keyboardadapted to control the operation and the functions of the device 200.

Referring to FIG. 14, the switches 234 of a key set 230 may also belocated under the light guide 100. The light guide 100 may be at leastpartially flexible, and one or more touch-sensitive sensors or switches234 may be positioned under the back side of a light guide 100. Theswitches 234 may also be proximity sensors positioned under the lightguide 100. The switches 234 may be implemented on a switch pad 232 or onthe back surface of the light guide 100. The light guide 100 may be anintegrated part of an illuminated key set 230.

Illuminating of the key set 230 comprises illuminating of a pattern 236associated with a function of said key-set 230. An out-coupling grating30 may be adapted to illuminate a pattern 236, which is associated witha function of a switch 234. The key set 230 may comprise a plurality ofilluminated patterns 236 and switches 234, wherein each pattern may beassociated with a function of a switch 234. Thus, it is not necessary toilluminate the switches 234 itself, and the switches 234 may be opaque.The pattern 230 may be e.g. a star pattern, a letter “Q”, “W”, “E”, “R”,a number, or another character. The patterns 236 may be implemented e.g.by printing ink on the out-coupling gratings 30, or by superposing apatterned mask on the out-coupling gratings 30. Also the perimeter, i.e.the shape of the out-coupling gratings 30 may correspond to a pattern236.

The device 200 may further comprise a battery, data processing and/ortelecommunications module. The device 200 may be portable. The device200 may comprise telecommunications capabilities. The device 200 may bee.g. a mobile phone, and/or a computer. Yet, the device 200 may be apersonal digital assistant (PDA), a communicator, a navigationinstrument, a digital camera, a video recording/playback device, anelectronic wallet, an electronic ticket, an audio recording/playbackdevice, a game device, a measuring instrument, and/or a controller for amachine.

For the person skilled in the art, it will be clear that modificationsand variations of the devices and method according to the presentinvention are perceivable. All drawings are schematic. The particularembodiments described above with reference to the accompanying drawingsare illustrative only and not meant to limit the scope of the invention,which is defined by the appended claims.

1-16. (canceled)
 17. A method for processing an input face of a lightguide, said light guide comprising an out-coupling grating implementedon a substantially planar surface of said light guide, said methodcomprising: cutting a waveguiding sheet to form a cut face which is atan angle cc with respect to said substantially planar surface, saidangle cc being in the range of 80 to 100 degrees, heating said cut face,and pressing a surface processing member against said cut face in orderto smooth or emboss said cut face.
 18. The method according to claim 17wherein the temperature of said cut face is at least temporarily duringsaid pressing higher than the glass transition temperature of thematerial of said sheet.
 19. The method according to claim 17 comprisingheating said cut face by one of: a hot gas flow and radiation.
 20. Themethod according to claim 17 comprising heating said surface processingmember by using a heater.
 21. The method according to claim 20 whereinthe temperature of said surface processing member is kept lower than themaximum temperature of the material of said sheet.
 22. The methodaccording to claim 17 comprising forming a diffraction grating on saidcut face, said grating being adapted to diffract more than 30% of lightinto diffraction order one or minus one.
 23. The method according toclaim 17 comprising forming at least two prisms on said cut face inorder to direct in-coupled light.
 24. The method according to claim 17wherein said sheet is cut by using a hot cutting edge, and said cuttingedge acts also as said surface processing member.
 25. A method fordistributing light by using a light guide comprising a substantiallyplanar waveguiding substrate, an input face to couple light into saidsubstrate, and an out-coupling grating to couple light out of saidsubstrate, wherein an angle between said input face and saidout-coupling grating is in the range of 80 to 100 degrees, said methodcomprising: coupling an input beam into said substrate through saidinput face to form an in-coupled beam, directing said in-coupled beam byusing at least one of: an input grating and prisms, and coupling lightout of said substrate by an out-coupling grating.
 26. The methodaccording to claim 25 further comprising lighting at least one of: a keyset and a display.
 27. A device comprising: a substantially planarwaveguiding substrate, an input face to couple light into saidsubstrate, and an out-coupling grating to couple light out of saidsubstrate, wherein an angle between said input face and saidout-coupling grating is in the range of 80 to 100 degrees, said inputface comprising at least one of: a grating structure and prisms adaptedto direct in-coupled light.
 28. The device according to claim 27 whereinsaid input face comprises an embossed input grating to diffract lightinto a first predetermined direction.
 29. The device according to claim27 wherein said input face comprises at least two embossed macroscopicprisms to refract light into a first predetermined direction.
 30. Thedevice according to claim 27, further comprising: a light source toprovide an input beam, and a key set, wherein said input face is adaptedto couple light of said input beam into said substrate to form anin-coupled beam propagating within said substrate, and said out-couplinggrating is adapted to couple light of said in-coupled beam out of saidsubstrate to form an output beam, said output beam being adapted toilluminate said key set, and said prisms comprising several inclinedprism faces adapted to direct said in-coupled beam.
 31. A deviceaccording to claim 30 wherein the surface of said input face beingsubstantially smooth such that at least one of: light-scatteringprotrusions and recesses cover less than 5% of the area of said inputface.